Low flame smoke

ABSTRACT

A smoke producing method and device of the present disclosure produces a non-incendiary, organic-polymerization based, smoke-producing reaction. The method of generating smoke comprises initiating a frontal polymerization reaction by heating a composition comprising a monomer compound that exothermically polymerizes upon initiation with an initiator compound and an initiator compound that initiates polymerization of the monomer compound present at a mass concentration that is at least five percent of the mass concentration of the monomer compound. The polymerization of the monomer compound is exothermic, and in one embodiment the concentration of initiator compound is at least five percent of the concentration of monomer compound. The smoke mainly comprises thermal decomposition products of the initiator compound.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/643,565 titled “Low Flame Smoke,” filed on May 7, 2012, the entirecontents of which are herein incorporated by reference.

GOVERNMENT LICENSE RIGHTS TO CONTRACTOR-OWNED INVENTIONS MADE UNDERFEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.W911SR-11-C-0084 awarded by the United States Army. The government hascertain rights in the invention.

BACKGROUND

Smoke generation devices generate smoke in military applications forsignaling, for marking target or landing zones, and for screening ofmovements. Devices for producing obscurant smoke for the battlefield aretypically either explosively-charged, meaning the devices use anexplosive charge to disperse fine particles, or chemically-reactive,meaning a chemical reaction generates smoke. Some chemically-reactivesmoke generation devices utilize inorganic materials that are activatedin a self-sustaining chemical reaction to produces smoke as a byproductof the heat generation. Examples of these smoke generation devices arethermite grenades and the HC (hexachloroethane), TA (terephthalic acid),and WP (white phosphorus, or red phosphorus) smoke grenades in thecurrent military inventory. The reactions in these devices have largefree energies of reaction, and are by necessity exothermic. As such, thereactions produce considerable heat and toxic, or hazardous, compounds.Typical smoke-producing reactions produce much more heat than isnecessary to sustain the reaction. The adiabatic flame temperatures ofthese materials greatly exceed 1000° C., which is one of the factorsthat leads to their incendiary characteristics.

Heat generation is an issue with either explosively-charged orchemically-reactive smoke generation devices. Traditional smokegeneration devices are incendiary and can set cloth, fuel, ammunitionand other combustibles on fire, and cause serious burns or death. Whatis desired is a smoke producing mixture that is capable of producingsmoke while minimizing the incendiary and chemical hazards of presentdevices.

SUMMARY

A smoke producing method and device of the present disclosure produces anon-incendiary, organic-polymerization based, smoke-producing reaction.In one embodiment, the method of generating smoke comprises initiating afrontal polymerization reaction by heating a composition comprising amonomer compound that exothermically polymerizes upon initiation with aninitiator compound, and an initiator compound that initiatespolymerization of the monomer compound present at a mass concentrationthat is at least five percent (5%) of the mass concentration of themonomer compound. In this embodiment, the smoke produced mainlycomprises thermal decomposition products of the initiator compound. Theinitiator may also decompose exothermically. The by-product that resultsfrom smoke generation in this embodiment is a solid material that willslowly degrade over time if exposed to outside conditions.

In a typical polymer reaction, the initiator concentration controls thechain length of the produced polymer. Also, in a typical polymerreaction, the initiator is consumed, chemically bonded to the polymericmolecules. In this type of smoke producing reaction the objective, at aminimum, is to decompose and volatilize initiator as well as additivesand/or portions of the monomer itself.

Frontal polymerization (FP) is a process in which the reactionpropagates directionally through the reaction vessel because of thecoupling of thermal transport and the Arrhenius-dependence of thekinetics of an exothermic reaction. Frontal polymerization is very muchlike a flame but propagating through condensed materials instead of agas. In frontal polymerization reactions, the components are premixed,but stable until initiated by an external source. For example, considera 2-part epoxy: as soon as the two components are mixed, an exothermicreaction is initiated). As another example, RTV type polymers willself-initiate once exposed to oxygen. The reactions developed hereoperate differently than either of these or similar types of examples.

Frontal Polymerization is a form of self-propagating high-temperaturesynthesis (SPHTS). Here the term “high-temperature” is used to indicatehigher than ambient temperature, but certainly lower in temperature thanpyrotechnic igniters used in current smoke grenades. In FP as in thecase of SPHTS the system will not start reacting until sufficient energyis applied to the material to get a reaction front propagating throughthe system. This self-propagating wave moves rapidly through the systemas long as sufficient heat is generated at the propagation front. Thus,these systems are inherently stable until a sufficient amount of energyis added to start the reaction. Materials with high heat capacity can beincorporated into the mixture. Thus, the system can be turned such thatthe heat released does not lead to excessive heating of the surroundingenvironment, thereby reducing incendiary hazards. In other words, theaddition of filler materials has the effect of reducing the fronttemperature and thereby reducing the incendiary hazard since the“excess” heat generated can be “absorbed” in the material itself and nottransmitted to the environment.

The reactants used in the smoke producing compounds disclosed hereinhave reaction temperatures in the range of 300-400° C., (However, asindicated above, the reaction temperature may be tuned to above ambientto 400 C). Thus, even with combustible, low heat capacity materials itis difficult for a device using these materials, particularly theexposed, exterior, material to get above the temperatures necessary tocause structural materials, such as wood, to combust. It is alsounlikely that if there were an accidental activation of a device duringstorage that other devices in the same container would ignite or thatother storage containers would be breached. In addition, the manufactureof devices with lower energetic materials is also much less hazardousthat current pyrotechnic based devices.

In a typical polymerization compound to make a polymer, the initiatorconcentrations are on the order of 1% or less by mass. Thisconcentration is expressed in polymer literature as 1 pph (parts perhundred of the monomer). As an example, a 10 gram sample with 20 pphinitiator and 10 pph fumed silica contains 10 grams of monomer, 2.0grams of initiator, and 1.0 grams of fumed silica. In experimentaltesting of the smoke producing compound of the present disclosure, itwas found that increasing the amount of initiator in the compoundincreased the amount of smoke produced.

Smoke production is caused by a decomposition of the monomer-initiatorpair in the smoke generation compound. The fact that smoke productioncomes from the monomer-initiator pairs has advantages. First, lowerreaction temperatures can be used because higher temperatures are notrequired to volatize a third component in the mixture. Since theinitiator is the source of the smoke in this embodiment, it is onlynecessary to have a sufficient reaction temperature to sustain theinitiator decomposition reaction. Also, a higher efficiency of smokeproduction can be achieved. Since the smoke is due to the initiator andno longer to a third component the “extra” mass was no longer necessary.The monomer itself may decompose, leading to additional smokeproduction.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be better understood with reference to the followingdrawings. The elements of the drawings are not necessarily to scale,emphasis instead being placed upon clearly illustrating the principlesof the disclosure. Furthermore, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 is a functional schematic of an exemplary test performed tomeasure the characteristics of a smoke producing sample.

FIGS. 2a-2f show a series of photographic measurements showing the smokedensity increase as increasing amounts of sample smoke producingmaterial are activated.

FIG. 3 is plot of the optical density versus time for a variety of smokeproducing compositions under test.

FIG. 4a is a schematic of a hypothetical mechanism of the decompositionpathway of the Luperox® 231.

FIG. 4b is a schematic of a hypothetical mechanism of the decompositionpathway of the mono- and di-function monomer impurities in thecommercial grade TMPTA.

FIG. 5 illustrates the results of an additional series of tests run withconcentrations approaching 50 pph.

FIGS. 6a-6c illustrate the tests performed to analyze initiation of asmoke producing reaction to measure the amount of smoke produced whenthe reaction was initiated from the front of a smoke producing samplecontained in a glass vial.

FIGS. 7a-7c illustrate the tests performed to analyze initiation of asmoke producing reaction.

FIG. 8 illustrates a plurality of cylindrical shapes tested in a seriesof trails of the smoke producing composition.

FIG. 9 is a photograph of a test setup from a series of tests of thesmoke producing composition spread out on a section of lumber.

FIG. 10 illustrates the visible absorption spectrum of the smokeproduced from the TMPTA-Luperox 231 reaction from the start of thereaction to about 20 minutes after is shown.

FIG. 11 illustrates the infrared absorption spectrum of the smokeproduced from the TMPTA-Luperox 231 reaction from the start of thereaction to about 9 minutes after the reaction.

FIG. 12 depicts a stacked disked embodiment of a smoke generatingdevice.

FIG. 13 depicts embodiment of a smoke producing device comprising asubstrate formed from a single sheet of material, rolled into a spiralshape.

FIG. 14 depicts a “stacked spiral” arrangement in which a plurality ofspiral substrates are stacked atop one another.

FIGS. 15a, 15b and 15c depict an embodiment of a smoke producing devicein which a plurality of cylindrical petals are arranged “concentrically”inside a cylindrical container that is hinged on one side.

DETAILED DESCRIPTION

The disclosure provides compositions for producing smoke. Variousembodiments of the compositions disclosed herein have advantages overpreviously known smoke-producing compositions; for example: low or noflame front (safe to use indoors, outdoors, and in training environmentswith flame hazards); low toxicity of the smoke and any non-smokeresidues; environmentally friendly (little to no residue or hazardousbyproducts); high packing density; high smoke yield/low agglomeration ofsmoke particles; easily aerosolized, rapid smoke generation (short timeconstant); good obscuration properties in the visible portion of theelectromagnetic spectrum; long smoke durations with appropriatebuoyancy; and good shelf life (i.e., after mixing components, themixture does not self-initiate polymerization).

In general, there are a minimum of two components—a monomer and aninitiator—required to achieve polymerization. In the present embodiment,the monomer provides the carbon compounds that will form the polymerchains and the initiator provides a mechanism to join the carboncompounds together. The baseline monomer used in the composition of thepresent disclosure is TMPTA (trimethylolpropane triacrylate). Othermonomers are possible and it is possible to combine other materials withthe monomer for various effects. For example, by combining TMPTA withdibutyl phthalate, a large amount of smoke can be generated, but thesmoke is not as buoyant as with TMPTA only. It may be possible todevelop a smoke with tailorable buoyancy—which is useful if it isdesired to reduce the duration of the smoke. Currently, in an enclosedenvironment, the smoke producing compound of the present disclosure canresult in smoke durations in excess of 20 min. Note that the monomer mayalso be a material with a backbone other than carbon; for example, theSilicon backbone in Silicone caulk or RTV sealant. In addition, theproduction of a polymer is not a necessity. The primary role of themonomer is that it provides the heat source so that the reactionproceeds in a timely manner. In Frontal Polymerization, as opposed toother polymerization mechanisms, the mixed monomer and initiator arestable until an external excitation source is added.

For example, by combining TMPTA with methyl benzoate, benzyl benzoate,and pentyl acetate, considerable amounts of smoke are produced but theyhave slightly less buoyancy than TMPTA only. This may result in theability to tailor the buoyancy. These materials are esters used as foodadditives/aromatics. An additional reason for employing TMPTA monomer inthe smoke mixture is that it is a good, high quality (purity),inexpensive monomer.

The baseline initiator for the smoke producing compound of the presentdisclosure is Luperox®-231(1,1-Bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane). Other initiatorsare possible but may have, or are shown to have, undesirable effects.For example, t-butyl peroxybenzoate may be used with good smokegeneration results. However, the benzoic acid byproducts areconsiderably more hazardous than the trimethyl cyclohexanes (TMCH)generated with the baseline initiator. The trimethyl cyclohexane smokeproduct or byproduct is not an acid or acid forming material. Accordingto the toxicity analysis the inhalation and LD50 thresholds of TMCH aremuch higher than for the currently used materials (HC and RP).

One embodiment of the smoke producing compound of the present disclosurerequires two other components: an ignition mechanism and a filler. Theignition (or initiation) mechanism used in the testing disclosed hereinwas a heat source. The heat source does not have to, but can, bepyrogenic. To date, Estes model rocket igniters, simple nichrome wireloops attached to voltage sources, hot air from a heat gun, solderingiron tips, open flame, focused intense light, have all been used toinitiate the FP reaction. This ignition mechanism list is notexhaustive. Other ignition mechanisms considered are: piezo devices thatmight be used to ignite something more pyrogenic such as cannon fuse,battery powered voltage sources for nichrome wire, etc. A mixtureincluding monomer and initiator will not self-initiate without anignition source—this contributes to the long shelf life and inertness ofthe material.

Ignition tests have been conducted with a 1″ conduction loop of 30 gaugenickel chromium (NiCr, or nichrome) wire with a resistance/unit lengthof approximately 0.5 Ohm/in. The wire was buried slightly under thesurface of the smoke producing composition (which is typically in gelform) and a current draw of approximately 1 Amp was sufficient toinitiate the FP reaction. Using Power, P=I²R, where I is the current inAmps and R is the resistance in Ohms, this yields an input Power of P=(1Amp)²(0.5 Ohm)=0.5 W.

In the current embodiment (for an application such as smoke grenadeusage), the filler provides a mechanism, or a matrix, for the smokemixture to have a shape other than that provided by its container (e.g.,a liquid or gas assumes the shape of its container, but a solid or a gelmay not). Fumed silica, kaolin (clay) powder, and powdered sugar haveall been used as fillers. Fumed silica has provided the best performancethe mass required is low, it has a high area-mass ratio which providessignificant thickening with a low thermal mass. This prevents it fromrobbing the reaction of the heat required for the reaction to propagate.Increasing the amounts of kaolin powder and powdered sugar have beenshown to rob the reaction of its necessary heat and reduce the amount ofsmoke.

There are other envisioned applications where the smoke mixture is leftas a liquid—so the filler/thickening agent might not be required ormight be detrimental to the application. An example of a situation inwhich the thickening agent is not required: A liquid smoke mixture iscarried on a military robot. If an individual approached too close tothe robot, the liquid would be sprayed onto a hot surface (i.e., hotplate or wire) located somewhere on the robot. This would generate asignaling/deterrent smoke. In addition, this might not require largetemperatures to initiate the reaction so that the smoke generationmechanism is not an incendiary hazard to the robot or to the localenvironment.

The primary mixture components of the smoke producing composition alsohave enough thermal conductivity that, if a point ignition source isapplied, the bulk mixture reactants may quickly convect the requiredreaction energy away from the reaction site and cause the reaction toquench itself. The very low thermal conductivity of fumed silica“insulates” the reaction region, preventing the heat of reaction or ofinitiation from convecting away too rapidly. When no filler is present alarge area heat source, such as a heat gun, may be required to injectsignificant heat into the mixture to overwhelm the convective heatlosses. Present experimentation has shown cases where, for all othermixture components held constant, increases in filler (fumed silica)have resulted in a higher absorption smoke. The filler may provide morenucleation sites for polymerization to initiate.

In one embodiment of the smoke producing compound, if X g of TMPTAmonomer is used, then greater than 0.1 X g of Luperox® 231 initiator,and greater than or equal to 0.1 X g of fumed silica filler are to beused. This mixture would be considered a “greater than 10 pph” mixture(greater than 10 parts initiator to 100 parts monomer). Note that theinitiator concentration may be allowed to approach infinity (i.e., nomonomer) and still generate smoke. The initiator may also decomposeexothermically. In comparison, ratios for standard reactions wherein thepolymerization product, not the smoke product, is desired, arecharacterized by initiator concentrations utilizing much less than 10pph—typically 0.01 pph-0.1 pph, but less than 1 pph.

The TMPTA (Trimethylolpropane triacrylate) is a trifunctional monomer.This means that there are three double-bond carbon ends associated witheach monomer molecule. Typical monomer-polymer system include compoundsthat have a single carbon double-bond along the monomer chain; ethylene,styrene, vinyl chloride. A single initiator molecule causes the breakingof the double bond and a monomer free radial to be formed. This monomerfree radical then reacts with other monomers and a polymer moleculebegins to grow. Termination of the process occurs when two free radicalscombine; either a second polymer free radical or the other half of theinitiator molecule. Polymer molecules of 1000 to 100,000 monomers arecommonly produced. One of the controlling parameters of the final chainlength is the number of initiator molecules added. Thus, typicalinitiator concentrations are a few hundredths to millionths of percent;high initiator concentrations yield low molecular weight polymermolecules. The heat generated from the polymerization process is due tothe breaking of the carbon double bond and the formation of a carbonsingle bond. This process releases 60 kJ of energy per mole of doublebonds. The process temperature of the reaction depends on the heatcapacity of the monomer molecules. Molecules such as poly(ethylene) C2H4have a much lower heat capacity than molecules such as styrene C8H10 andhave much higher reaction temperature since they both have a singledouble-bonded carbon that participates in the reaction.

Experimental Testing

The addition of “excess” initiator, in this case Luperox® 231Di-(tert-butylperoxy)-3,3,5-trimethylcyclohexane)), to a trifunctionalmonomer is against all polymerization practice because it increases theamount of smoke and decreases the quality of the resultant polymer. Infact, the more initiator is added, the poorer the strength of theresultant polymer, because there are more voids, more fractures, etc.During the course of this work it was not clear, until experimentaltests were performed, that the polymerization reaction would even occuras increasing amount of initiator were added to the monomer. Increasingthe initiator amount beyond the minimum necessary to sustain thepolymerization reaction, likely causes an excessive number ofpolymerization reactions to occur simultaneously in a confined space.The distinct polymers formed by these multiple polymerization reactionswill not necessarily bond with other polymers to form longer polymers.The result is that shorter than normally desired polymer chains areformed, resulting in a far weaker polymer product. As the initiatorconcentration is increased excessively, the polymer product has muchshorter chains and is far weaker.

A series of preliminary experiments were conducted with initiatorconcentrations from 1 to 15 pph (parts per hundred of monomer). Thesepreliminary tests qualitatively indicated that higher initiatorconcentrations resulted in increasing smoke yields. More importantly,these tests indicated that high initiator concentrations did notadversely affect the rate of the polymerization process and thatsufficient heat was generated for the initiator to decompose into avisible smoke.

FIG. 1 is a functional schematic of an exemplary test performed tomeasure the characteristics of a smoke producing sample 101 in a chamber100. The chamber 100 was substantially one (1) cubic foot in volume(11″×12″ by 10″). Specifically, a Fisher Scientific® Dry Box was used asan air tight chamber 100 in this test. An FP reaction of the sample 101was remotely initiated via a wire 108 extending through the chamber walland to a power source (not shown). A fan 106 inside the chamber 100circulated the smoke (not shown) produced by the reaction. Visiblespectra measurements were taken with an Ocean Optics HR2000 UV-Visspectrometer 102. The optical cell (not shown) was a Starna 34-SOG-10010 cm cell. Infrared spectra were determined with a Nexus470 FTIR 103using a 4″ pathlength cell (not shown) with KBr windows (not shown).

The chamber 101 comprised a transparent window 107 to allow visualaccess to the sample under test for viewing the smoke and measuringsmoke parameters. A vent hood 104 collected fumes from the test and avent 105 vented fumes outside of the building.

In a similar test of the smoke producing sample, a 50 ft³ PVC andplastic wrapped chamber (not shown) was constructed. Two clear plasticwindows 204 (FIG. 2a ) on the chamber 200 (FIG. 2a ) provided foroptical measurements and visualization of the smoke production.

A series of experiments were completed in both the 1 ft³ and 50 ft³chambers to test the limits of smoke production with increasinginitiator concentration. Measurements of smoke production versusinitiator concentration from 5 to 50 pph have been made in the 1 ft³chamber and from 5 to 25 pph in the 50 ft³ chamber. For tests in boththe 1 ft³ and 50 ft³ chambers optical transmission measurements (I/I₀)were made versus time using a 633 nm laser and Newport laser powermeter. From these tests it was determined that increasing the initiatorconcentration to at least 25-30 pph gave a good smoke productionreaction and that increasing to 50 pph would continue to produce moresmoke. Tests were run to quantify the amount of material necessary toproduced a dense enough smoke for obscuration. A series of tests usingdifferent sample weights with 25 pph starting material versus opticaldensity were run in the 50 ft³ chamber. The amount of material wasincreased from 5 to 25 grams of monomer (all with 25 pph of initiator);this corresponds to 0.1 to 0.5 grams of monomer per ft³ of chambervolume.

FIGS. 2a-2f show a series of photographic measurements showing the smokedensity increase as increasing amounts of sample smoke producingmaterial are activated. A laser power meter 201 measured opticaltransmission of smoke in the chamber 200. Tape 203 defined a rectangulartransparent window 204. Two tape strips 202 were mounted horizontally onthe opposite inside side wall of the chamber. As can be seen in FIG. 2a, which illustrates the chamber 200 before a smoke producing reaction isinitiated, the tape strips 202 are clearly visible through the window204. However, as smoke concentration increases, as shown in FIG. 2b , inwhich the smoke density is 0.10 grams monomer per cubic foot, the tapestrips 202 become less visible. The beam 207 from the laser power meter201 is clearly visible in FIG. 2 b.

In FIG. 2c , which illustrates a smoke density of 0.15 grams monomer percubic foot, the tape strips 202 are invisible. In FIG. 2d , the smokedensity is 0.20 grams monomer per cubic foot. In FIG. 2e , the smokedensity is 0.25 grams monomer per cubic foot. In FIG. 2f , the smokedensity is 0.30 grams monomer per cubic foot.

It is notable that the testing illustrated in FIGS. 2a-2f was performedindoors in plastic containment chambers. This highlights thenon-incendiary characteristic of the reaction. The smoke does have anodor to it so the chamber needs to be vented outside. However, anunpleasant odor could be advantageous in some situations where a “stinkbomb” might be desired.

FIG. 3 is a plot of the optical density versus time for the same mass ofmaterials from testing performed in 50 ft3 chamber. This figure showsthat after about 0.15 grams of starting monomer per cubic foot (gm/ft³),the optical density drops below 0.1. Comparing the results of FIG. 2cwith FIG. 3 at 0.15 gm/ft³ the smoke density is almost sufficient tototally obscure the reference tapes 202 (FIG. 2c ) on the opposite wall.As the sample mass increases up to 0.3 gcf the smoke density and itsobscurant ability clearly increase.

The photographic series FIGS. 2a-2f illustrates a quirk of the laserbeam visibility: with increasing smoke density, the laser beam 207actually seems brighter and more visible. This result is also shown inthe data of FIG. 3. The measured optical density for starting samplemass of greater than 0.15 gcf is actually greater than for 0.15 gcfitself, while it is clear from the photographs in FIG. 2c-2f that thesmoke is denser. This higher measured optical density is likely due to amultiple scattering phenomena competing with the initial beamabsorption/scattering. Note also from FIG. 3 that the duration of thesmoke (at least in this controlled environment, i.e., in the absence ofdriving winds) is considerable.

Decomposition Products

The starting monomer and initiator in the exemplary testing was TMPTAand Luperox® 231. The expected decomposition products have been analyzedboth through a literature review and via Gas Chromotograph-MassSpectrometer (GC-MS) analysis of the smoke products. The literaturereview lists as the decomposition products:

-   -   a. 3,3,5-trimethylcyclohexane,    -   b. 2,4,4-trimethylcyclohexane,    -   c. Trimethylcyclopentane    -   d. t-butyl alcohol,    -   e. acetone,    -   f. ethane, and    -   g. carbon dioxide.

Experimental GC-MS analysis essentially confirmed the literature resultsbut showed only three components in the smoke:

-   -   a. 3,3,5-trimethylcyclohexane,    -   b. 2,4,4-trimethylcyclohexane, and    -   c. t-butyl alcohol.

Neither acetone nor trimethylcyclopentane were detected. The molecularweights and melting and boiling points of some of the decompositioncomponents are listed in Table 1 below. Acetone and Tert-butyl alcoholare gases room temperatures and the trimethylcyclohexane is liquiddroplets at room temperature.

TABLE 1 Molecular weights and melting and boiling points of Luperox ®231 decomposition products Molecular Melting Boiling DecompositionProduct Weight Point Point Vapor Species [g/mole] [° C.] [° C.]1,3,5-trimethylcyclohexane 126.24 −49.7 138.5 Acetone 58.08 −95 56.2Tert-butyl alcohol 74.12 25.2 82.2

FIG. 4a is a schematic of the decomposition pathway of the Luperox® 231and FIG. 4b is a schematic of the decomposition pathway of the mono- anddi-function monomer impurities in the commercial grade TMPTA. In thisschematic, dotted lines are cleavage.

From the GC-MS analysis of the smoke produced, the reaction products aretrimethylcyclohexane and t-butyl alcohol. The reaction products of themonomer decomposition are not seen in the smoke but may affect itsinfrared absorption properties.

Total Sample Mass Loss During Smoke Production

A series of tests were performed to measure the mass loss of the samplesmoke generation compound versus the amount of initiator used in thecompound. These tests were performed to confirm that the majority of theinitiator was decomposing, and this expectation was confirmed. For thehigher initiator concentrations and for thin (<⅛″) sample thickness,there was more mass loss than just the initiator itself. Thesignificance of sample thickness is discussed further below.

A series of tests was also performed to determine the mass loss over awider initiator concentration range, and the initiator concentration wasvaried from 1 pph to 30 pph. The fumed silica (thickening agent) contentwas held constant at 10 pph. The starting TMPTA monomer was 2 grams andthe mass of the initiator was varied from 0.02 to 0.60 grams. Two tothree samples were run for each mixture composition. The results ofthese tests are presented in Table 2 below.

TABLE 2 Percent mass loss of monomer-initiator-filler mixtures versusthe initial initiator concentration. Initiator Concentration [parts perhundred] 1 5 10 20 30 Percent mass loss 0.5-1 4.0-5.2 9.8-13.4 22-3233-49 (number of samples) (2) (3) (3) (3) (2)

As can be seen from Table 2, from about 1 to 5 pph of initiator, themass loss was approximately proportional to the amount of initiatoradded. At higher initiator concentrations (greater than 10 pph) thetotal mass loss was greater than the initiator mass. The additional massloss—resulting in more smoke—is considered to be due to a decompositionof mono-functional, and di-functional “impurities” that are present inthe commercial grade TMPTA. The additional mass loss could be due to adecomposition of the tri-functional TMPTA itself, but this is consideredto be unlikely.

FIG. 5 illustrates the results of an additional series of tests run withconcentrations approaching 50 pph. Note that it is unclear whether thatthe mass loss rate is decreasing at the 50 pph (50%) point. Thisindicates that it is desirable to perform additional tests withinitiator concentrations greater than 50 pph.

The internal temperature of 5 gram samples of the mixed compound wasmeasured in order to better understand the safety, and non-incendiary,characteristics of the frontal polymerization reaction. In initiatorconcentrations of less than 5 pph, the internal sample temperature was100-200° C. At initiator concentrations from about 15 to 30 pph, theinternal temperature increased to 300-350° C. This temperature is likelysufficient to lead to some decomposition of the monomer itself, whichmay be helped by the appreciable excess of initiator.

Effect of Sample Layer Thickness and Geometry on Smoke Production

A series of tests was performed to determine the effect of aspect ratio(width v. length at fixed heights) of the sample versus the amount ofsmoke produced. These tests were conducted under three testing/operatingscenarios, 1) front and rear initiation of the reaction, 2) cylindricalsamples of varying aspect ratio, initiated from the top “free” surface,and 3) rectangular samples of varying aspect ratios. Test geometries 1)and 2) were conducted in the one ft³ test chamber and the third seriesof tests were conducted in the 50 ft³ chamber. The sample smokeproducing compound was 10 pph Luperox® 231 and 10 pph filmed silicafiller.

Tests of Front Versus Rear Reaction

FIGS. 6a-6c illustrate the tests performed to analyze initiation of asmoke producing reaction to measure the amount of smoke produced whenthe reaction was initiated from the front, expanding portion, of thesample contained in a glass vial. In FIG. 6a , the sample 600 isdisposed near an open front end 601 of a glass vial 602. In FIG. 6b ,the sample 600 has just been ignited. FIG. 6c is a wider view of thesample 600 after the smoke has expanded. The smoke was close toneutrally buoyant and filled the test chamber in an amount that would beexpected, given the size of the sample.

FIGS. 7a-7c illustrate the tests performed to analyze initiation of asmoke producing reaction to measure the amount of smoke produced whenthe reaction is initiated from a sample disposed in the rear,constrained, portion of a glass vial. In FIG. 7a , the sample 700 isdisposed near the rear end 701 of a glass vial 702. In this series oftests, as shown in FIGS. 7b and 7c , any hot smoke vapors have to travelthrough the unreacted portion of the sample before reaching the open end703 of the vial 702. The resultant smoke was denser than the surroundingair and tended to sink to the bottom of the test chamber. In both of thetests illustrated in FIGS. 6 and 7, the frontal polymerization reactionproceeded to completion.

Tests of Cylindrical Samples of Varying Aspect Ratios

The second series of trials tested a constant volume of material inthree cylinder shapes with bores of different aspect ratios, 2:1, 1:1,1:3, and 1:5, as illustrated in FIG. 8. The cylinder bores weregenerated by drilling holes in a Delrin puck. A syringe was used toplace the samples in the bore holes. These tests showed that the 2:1aspect ratio sample had the most smoke production; the 1:3 and 1:5aspect ratio tests produced a minor amount of smoke. The 2:1 aspectratio test produced a typical amount of smoke. The test results arereported in Table 3 below. In each of these tests, the reaction wasinitiated at the top of the sample with enclosed sides and bottom. Theconclusion from these tests is that a low aspect ratio of height todiameter is desirable.

TABLE 3 Optical transmittance of smoke produced for various aspect ratiocylindrical samples Sample Aspect Ratio Sample Diameter OpticalTransmittance [diameter to height] [inches] [I/I0] 2:1 1 0.20 1:1 ⅝ 0.8 1:3 ½ 0.97 1:5 ¼ 1.0-no signal lossTests of Rectangular Samples of Varying Aspect Ratios

FIG. 9 is a photograph of a test setup from the final series of tests,which were conducted with 10 gram samples (20 pph initiator, 10 pphsilica), spread out on a section of lumber 900. Selected thicknesslumber guide rails 901 were spaced about one inch apart, the guide railswere varied from 3/16 inches in height, to ¼″ in height to ½″ in height,and the sample 902 (shown after the reaction) was spread out to roughly1.5 to 4 inches long between the guide rails. Note that the in FIG. 9lumber shows no signs of combustion and in spite of the fact that it hasbeen used for several dozen tests. The measured optical density valuesare given in Table 4 below. These results confirm that the layerthickness play a critical role in the efficiency of smoke produced.

TABLE 4 Optical transmittance measurements versus aspect ratio andsample thickness for fixed mass samples. Sample Sample Sample OpticalAspect Ratio Thickness Length Transmittance [height to length] [inches][inches] [I/I0] 1:20 3/16 ~4 0.10 1:12 ¼ ~3 0.25 1:3  ½ ~1.5 .98-nosignal lossTest with Monomers and Initiators Other than TMPTA and Luperox® 231

A series of tests were conducted with TMPTA and initiators other thanLuperox® 231 and tests of monomers other than TMPTA to confirm that thesmoke production was due to the decomposition of the Luperox® 231 and toconfirm the effectiveness of TMPTA as the monomer. These tests were onlyrun for qualitative, rather than quantitative smoke productionassessment. The mixture composition was 10 pph initiator and 10 pphfumed silica. Table 5 shows the results of these tests.

TABLE 5 Monomer-Initiator combinations tested for their qualitativesmoke production ability. Initiators t-butyl Monomers Luperoxe ® 231peroxybenzoate TMPTA (Trimethylolpropane Good smoke-Control Similar toControl triacrylate) sample TMPTA + dibutyl phthalate Good or bettersmoke- No Test smoke sinks PETA (Petaerythritol Poor smoke Poor smoketriacrylate) DTMPTA Poor or no smoke No smoke (Di(trimethylolpropane)triacrylate)

The results in this table highlight the fact that the Luperox® 231/TMPTAinitiator/monomer combination is rather unique in its ability to producelarge volumes of smoke. The t-butyl peroxybenzoate initiator did producegood quality of smoke. However, one of its reaction products would bebenzoic acid. Thus, a smoke from this initiator would have a much highertoxicity than the methylcyclohexanes from Luperox® 231. TheTMPTA+dibutyl phthalate mixture did produce a good quality, albeitsinking, smoke.

Visible Optical Signatures

FIG. 10 illustrates the visible absorption spectrum of the smokeproduced from the TMPTA-Luperox 231 reaction from the start of thereaction to about 20 minutes after is shown. The data was taken usingthe one ft3 chamber that was connected to the Ocean Optics spectrometerthrough flow-ports installed in the back of the chamber. This figureshows that the smoke produced has a uniform absorption across the(entire) visible spectrum from 300-1000 inn. Thus, it evenly scattersall the visible wavelengths. It can also be seen in the figure that thesmoke has a persistence of at least 5 minutes. From this data and fromother tests this indicates that the particle sizes are in a range wherethere is not rapid sedimentation of the particles or droplets.

Infrared Optical Signatures

FIG. 11 illustrates the infrared absorption spectrum of the smokeproduced from the TMPTA-Luperox 231 reaction from the start of thereaction to about 9 minutes after the reaction. The data was taken usingthe 1 ft³ chamber that was connected to the Nexus 470 FTIR systemthrough flow-ports installed in the back of the chamber. The infraredcell has KBr windows. The infrared spectrum has unique peaks associatedwith the trimethylcyclohexane, t-butyl alcohol, and acetone produced inthe reaction. The infrared peak from a human body is centered around 10μm; indicating that the current version of this smoke is not an infraredobscurant for humans. The absorption peaks at approximately 6, 7 and 8μm indicate that the smoke has obscurant properties for 225, 150, and100° C. bodies. No efforts were made during the Phase I research tomodify the reaction products to make the smoke obscure humans.

Toxicity of Decomposition Products

The toxicity of the decomposition products has been analyzed from theMSDS data that is available for the initiator decomposition products:trimethylcyclohexane, tert-butyl alcohol, and acetone. Values for theknown decomposition products of our formulation and current inventorygrenades are given in Table 6 below. While excessive exposure to acetoneand tert-butyl alcohol should be avoided, these compounds are theprimary component of many household products such as nail polishremover. Table 6 below shows that the decomposition products of thesmoke producing formulation disclosed herein are substantially lesstoxic or reactive than presently used compounds. (Hexachloroethane andphosphoric acid are included as reference materials.)

TABLE 6 Toxicity and workplace exposure data for Luperox ® 231decomposition products. LD50 Exposure limit [mg/kg] [(mg/kg)/time-hrs]notes trimethylcyclohexanes No data No data Chronic effect on humans-TWA 2000 mg/m³ toxic to lungs methylcyclohexane 2,250-oral   7613vapor-4 hours Chronic effect on humans- toxic to lungs tert-butylalcohol 2,743-oral 10,000 vapor-4 hours may cause reproductive systemdamage acetone  3000-oral 44,000 vapor-4 hours may cause CNS damagehexachloroethane 4,900-oral No data but known Confirmed animal (M8 HC)respiratory irritant carcinogen, very toxic to TWA-10 mg/m3 aquaticlife-long lasting Terephthalic acid 3200 TWA-10 mg/m³ Chronic toxicityto (M83 TA) multiple organ systems phosphoric acid  1550-oral   850vapor-1 hour TLV-1 mg/m³ LD50 = Median Lethal Dose TWA = Time WeightedAverage TLV = Threshold Limit Value

Questions have been raised as to whether adding oxiders to the mix wouldit speed up the reaction and make smoke faster. The composition is notincendiary, and adding (inorganic) oxidizers to the mix may cause it tostart a fire, which would be undesirable. Therefore, the compositionavoids inorganic oxidizers. The smoke in the composition is producedfrom the decomposition of the initiator in the composition, which can bethought of as an/the oxidizer. The composition differs from currentlyknown formulations in that it is this “oxidizer” that makes the smoke.Adding an inorganic oxidizer would likely cause the smoke production todecrease.

The desired smoke production requires approximately 0.020 grams ofmaterial per cubic foot of obscured volume when viewed through a 10 mthick smoke screen. For a 5 m thick smoke screen 0.04 grams/cu. ft. ofmaterial are required. The Obscurant factor is constant across thevisible spectrum, and has infrared absorption in specific wavelengthranges. Assuming ideal and complete reaction efficiency, for a 300 m³ (3m×10 m×10 m or 10,600 ft³) obscured volume, approximately 200 cm³ ofmaterial is projected to be required, representing a deviceapproximately 4 inch in height and 2 inches diameter; without casing,fuse or ignition source. Analysis of the mechanism of smoke productionindicates a strong potential that a smoke could be produced with0.010-0.015 grams of material per cubic foot of required coverage. Thecasing and fusing requirements will result in a final device size ofgenerally 5 inches in height and about 3 inches diameter; whichrepresents devices currently in the inventory.

It is unlikely that the local oxygen concentration has any effect on theamount of smoke produced. Based upon the decomposition mechanism of theLuperox® 231, oxygen is not required. It is currently unknown whetherextra mass loss from the mono- or di-functional monomers requires oxygenor not.

FIG. 12 depicts an embodiment of a smoke generating device 1100 usingthe compound disclosed herein. In this “stacked disk arrangement,” thesmoke generating compound (not shown) is applied to disks 1101, 1102,1103, 1104 and 1105 stacked atop one another. Although five (5) disks1101-1105 are shown in FIG. 11, this number of disks is illustrated forexplanatory purposes; a smoke generating device 1100 may comprise 10-30stacked disks, or more or fewer, as desired.

In this embodiment, each disk 1101-1105 is formed from non-woven fiber,such as a plastic fiber similar to Scotch Brite® pads or a plasticBrillo® pad, or fiberglass. The disks 1101-1105 may also be formed fromother materials with a high surface area for maximizing thecomposition's exposure to oxygen during the smoke-producing reaction.

An ignition wire 1106 extends through openings 1107 in the disks1101-1105 for initiating the reaction. In other embodiments, theignition wire 1106 may be “woven” into the fiber comprising the disk.

Wires 1108, 1109, 1110, and 1111 extend between adjacent disks. In thisregard, wire 1108 extends between disk 1101 and disk 1102; wire 1109extends between disk 1102 and disk 1103; wire 1110 extends between disk1103 and disk 1104; wire 1111 extends between disk 1104 and disk 1105.

In some embodiments, insulators (not shown) are disposed betweenadjacent disks to isolate each disk from the remaining disks, to preventthe disks from sticking together.

FIG. 13 depicts an embodiment of a smoke producing device comprising asubstrate 1300 formed from a single sheet of material, rolled into aspiral shape as shown. The substrate 1300 may be formed from thematerials discussed above with respect to FIG. 12. An ignition line 1301extends through the substrate 1300.

FIG. 14 depicts a “stacked spiral” arrangement in which a plurality ofspiral substrates 1400 like those discussed above with respect to FIG.13 are stacked atop one another. Each substrate comprises an ignitionline 1401.

FIGS. 15a, 15b and 15c depict an embodiment of a smoke producing devicein which a plurality of cylindrical petals 150, 151 and 152 nestedinside a cylindrical container 153 that is hinged on one side via ahinge 154. FIGS. 15a and 15b depict the container 153 before the smokeproducing ignition is initiated, and FIG. 15c depicts the container 153after the ignition has begun. Although three petals 150, 151, and 152are depicted in the illustrated embodiment, more or fewer petals areemployed in other embodiments.

The ignition sequence causes the container 153 to be split so that itopens up along a hinge line 155 of the container 153. The concentricallyarranged petals 150, 151 and 152 are ignited and split along one side sothat they “open up” like a blooming flower. Each of the petals 150, 151and 152 may be formed from the materials discussed with respect to FIG.12 above.

What is claimed is:
 1. A low-temperature method of generating smoke, themethod comprising initiating a frontal polymerization reaction byheating a composition comprising a monomer compound that exothermicallypolymerizes upon initiation with an initiator compound and an initiatorcompound that initiates polymerization of the monomer compound presentat a mass concentration that is 5% or more of the mass concentration ofthe monomer compound, wherein the polymerization of the monomer compoundis exothermic, wherein the concentration of initiator compound is 5% ormore of the concentration of monomer compound, and wherein the smokemainly comprises thermal decomposition products of the initiatorcompound; wherein the initiator compound is t-butyl peroxybenzoate. 2.The method of claim 1, wherein the monomer compound is TMPTA.
 3. Themethod of claim 1, wherein the monomer-initiator combination will notself-initiate or self-polymerize.
 4. The method of claim 1, wherein thereaction is self-sustaining once the reaction has been initiated.
 5. Themethod of claim 1 comprising heating the composition by running anelectric current through a conductive wire in contact with thecomposition.
 6. The method of claim 1, comprising heating thecomposition by running an electric current through a nickel-chromiumwire in contact with the composition.
 7. The method of claim 1, whereinthe mass/mass ratio of initiator compound to monomer compound is about1:1-20:1.
 8. The method of claim 1, wherein the composition comprises afiller agent.
 9. The method of claim 1, wherein the compositioncomprises an infrared-opaque agent.
 10. The method of claim 1, whereinthe composition is in a non-fluid form having a first dimension and asecond dimension, and the ratio of the first dimension to the seconddimension is less than
 1. 11. The method of claim 1, wherein thecomposition does not contain a significant amount of an inorganicoxidizer.
 12. The method of claim 1, wherein the composition furthercomprises dibutyl phthalate.
 13. The method of claim 1, wherein thecomposition further comprises fumed silica.